The components used in optical networks are often complex structures, individually fabricated for specific applications of use. Though complex overall, many of these components are formed of relatively simple individual optical devices combined to achieve complex functionality. Just as the advent of semiconductor logic gates facilitated the creation of the microprocessor, the development of simple optical devices performing functions such as coupling, splitting, and constructive/destructive interference allows system designers to form increasingly more complex optical circuits.
Of the various basic optical structures, signal splitting/combining is one of the most important. Generally, splitting/combining is achieved through either direct or indirect coupling means. Indirect coupling, for example, relies upon evanescent field coupling through two close proximity waveguides, one being a source waveguide. Direct coupling instead involves bringing an input waveguide (or propagating medium) in direct physical contact with one or more output waveguides. Y-branches and multimode interference (MMI) couplers are two examples of direct coupling structures that can be used to split an optical signal or combine multiple optical signals.
Y-branches are the most common direct coupling structures for splitters/combiners. Planar lightwave circuits (PLCs), integrated optics and lasers have all been shown with Y-branches. Y-branches are currently used as power splitter/combiners in branching tree configurations and in interferometers. The former are typically stand alone structures that are not phase sensitive, whereas the latter are used in filter designs (e.g., channel interleavers in dense-wavelength division multiplexing (DWDM) systems) and are phase sensitive. Both types of Y-branches have their limitations. The latter phase-sensitive devices, for example, are sensitive to small variations in device performance, and, for a splitter, a small change in the splitting ratio may render an entire optical device inoperable.
Y-branches are formed of a straight input waveguide (for receiving an input signal) and two output waveguides that meet at the linear waveguide. Where the two output waveguides meet, a sharp inner edge is formed forming equal branching angles for the two output waveguides. The two output waveguides are typically S-shaped waveguides branching off from this sharp inner edge.
Unfortunately, state of the art Y-branches lose a sizeable amount of input energy due to limitations in device fabrication. Y-branch fabrication is a lithographic process in which high-quality lithography equipment, such as E-beam lithography equipment. Even with such equipment, it is difficult to fabricate well-aligned and symmetric output waveguides especially at the smaller sizes. Even if perfect alignment were to be achieved in one device, reproducing that alignment across a batch of fabricated devices is not likely.
To avoid the cost associated with such high-quality lithography equipment, lower quality lithography techniques are used. Of course, there is a quality tradeoff, and the equipment results in non-ideal Y-branch fabrication—a problem most noticeable at the inner edge where the two output waveguides of the Y-branch are to meet.
To facilitate more affordable lithography techniques, a few have used blunts to eliminate the splitting mismatch that occurs with poor-quality inner edges. Blunts, therefore, can correct for fabrication defects batch-to-batch or device-to-device. Yet, though useful in correcting for splitting ratio errors, current blunt designs result in a measurable overall loss of input signal power. For example, each output branch in a 50/50 splitter receives much less than the ideal 50% of the input power, due to blunt inducted losses. The losses are in part due to mode confinement of the input signal to the middle of the blunt section, i.e., between the two output waveguides and not at their input faces limitations. In short, blunts eliminate inner-edge error that results from fabrication, but do so by sacrificing signal power. As provided in the foregoing, there is a tradeoff with fabrication techniques and device performance for known Y-branch structures.